Hydrocarbons, such as oil and gas, are produced from cased wellbores intersecting one or more hydrocarbon reservoirs in a formation. These hydrocarbons flow into the wellbore through perforations in the cased wellbore. Perforations are usually made using a perforating gun loaded with shaped charges. The gun is lowered into the wellbore on electric wireline, slickline, tubing, coiled tubing, or other conveyance device until it is adjacent to the hydrocarbon producing formation. Thereafter, a surface signal actuates a firing head associated with the perforating gun, which then detonates the shaped charges. Projectiles or jets formed by the explosion of the shaped charges penetrate the casing to thereby allow formation fluids to flow through the perforations and into a production string.
Tubing conveyed perforating (TCP) is a common method of conveying perforating guns into a wellbore. TCP includes the use of standard threaded tubulars as well as endless tubing also referred to as coiled tubing. For coiled tubing perforating systems, the perforating guns loaded with explosive shaped charges are conveyed down hole into the well connected to the end of a tubular work string made up of coiled tubing. TCP can be particularly effective for perforating multiple and separate zones of interest in a single trip. In such situations, the TCP guns are arranged to form perforations in selected zones but not perforate the gap areas separating the zones.
Some conventional system for perforating multiple zones includes perforating guns that are fired using a pressure activated firing head. Each firing head is set to actuate upon detecting a preset fluid pressure. During operation, the operator increases the pressure of the wellbore fluid in the well by activating devices such as surface pumps. The firing heads, which are exposed to the wellbore fluids, sense wellbore fluid pressure, i.e., the pressure of the fluid in the annulus formed by the gun and the wellbore wall. Once the pre-set value of the annulus fluid pressure is reached for a firing head, the firing head initiates a firing sequence for its associated gun.
In some instances, pressure variations, such as pressure spikes associated with the firing of a perforating gun, can interfere with the pressure-activated firing heads for these systems. The present disclosure addresses the need to protect pressure-activated firing heads from undesirable pressure variations as well as other drawbacks of the prior art.